This invention relates, in general, to refining of steel, and more particularly, to an improvement in the basic oxygen process, i.e. a process wherein molten steel contained in a vessel is refined by top blowing oxygen into the melt. More specifically, this invention is directed to a method for increasing the nitrogen content of steels made by the basic oxygen process.
The manufacture of steel by the basic oxygen process, also referred to as the BOP or BOF process, is well known in the art. When low-carbon steel is made by this process, its dissolved nitrogen content is subject to wide variations. Certain grades of steels have specifications requiring a low nitrogen content, and methods have been devised to achieve this, for example, as disclosed by Glassman's U.S. Pat. No. 3,769,000 and Pihlblad's U.S. Pat. No. 3,307,937.
On the other hand, some steels have specifications calling for high nitrogen contents, hence methods of increasing the nitrogen content have also been devised. Many of these methods require a separate nitrogen addition step after completion of the conventional decarburization step. Examples of such methods are shown in U.S. Pat. No. 2,865,736, U.S. Pat. No. 3,402,756, and U.S. Pat. Nos. 3,356,493; 3,322,530; and 3,230,075.
U.S. Pat. No. 3,180,726 discloses blowing the melt with pure nitrogen or nitrogen together with an inert gas and adding a stabilizing or fixing element after the blow. This method, however, does not allow the steel maker to adjust the nitrogen content of the steel independently, i.e. without altering the composition of the melt by adding other alloying elements. All of the above methods have the disadvantage of requiring an additional step after oxygen refining, thereby increasing the time required to make each heat of steel. Furthermore, some require the addition of other elements in order to fix the nitrogen in the melt, while others require complex teeming apparatus.
Still another approach used by the prior art has been to increase the nitrogen content of the melt during decarburization. U.S. Pat. No. 3,754,894 shows how the nitrogen content of steels may be increased during decarburization provided, however, that the decarburizing gases and nitrogen are injected from beneath the surface of the bath. This method is not easily combined with the BOF process wherein all gases are injected from above the surface of the melt. If nitrogen gas is merely blown into a basic oxygen vessel from above the bath during conventional decarburization practice, the results will not be reproducible, and the aim nitrogen content will be achieved only fortuitously.
It has not been possible, prior to the present invention, to make steels having high nitrogen contents by the basic oxygen process without performing a separate step after decarburization and/or adding elements to the melt in addition to nitrogen.